1. Field of the Invention
The present invention relates to an optical information reproducing apparatus for reproduction of data recorded in sectors of a recording medium.
2.Description of the Prior Art
FIG. 1 is an external front view of a known optical disk. The optical disk denoted by the numeral 1 is provided with guide tracks 2 formed thereon in a spiral or concentric arrangement for optical detection and for serving as a guiding path(s), whereby the recording tracks are arranged at high density and discrete writing and erasing of fragments of data can be ensured. The recording is carried out by forming pits or providing variations in reflectivity or transmittance using a laser beam of less than 1 .mu.m in diameter directed to a recording layer provided on the guide track 2.
For recording a variable length digital information data, the track is separated into a plurality of sectors to enhance the recording efficiency, allowing the data to be recorded and reproduced on a sector by sector basis.
FIG. 2 shows a graphic representation of a sector. Each sector comprises an ID field 4 containing track and sector address information and a data field 5 for the recording and reproducing of data.
A data format of the data field 5 is shown in FIG. 3. The data field 5 comprises a pull-in signal 6 for pulling in with a phase locked loop, marks 7 for identification of the leading ends of data (which will be referred to as data marks hereinafter), and segments of data 8. For demodulation of the data, one of the data marks 7 is detected from a reproduced signal to produce an enable signal which announces the presence or absence of data. The enable signal is then examined to determine whether it is active or non-active, for thereby determining whether the data has been demodulated, and also, for verifying the end of demodulation.
On the other hand, if there is any flaw, dust, or damage on the substrate, recording layer, or protective layer of the optical disk, the reproduced signal induces drop-out. Since both the size of each recording pit and the track pitch on the optical disk are as small as about 1 .mu.m, the rate of actual operational error is practically not less than a range between 10.sup.-4 and 10.sup.-5 and also, the drop-out of the reproduction signal will occur at random or in an extensive burst-like form.
Additionally, the recording layer of the optical disk is intended to involve a limited number of recording operations and will gradually experience an increase in the actual error rate before finally becoming unusable for demodulation.
When examining whether a given sector is recorded or when retrieving an unrecorded sector, the leader of the data must be detected. When it is found out that a corresponding demodulation circuit remains inactivated, the sector is judge unrecorded. However, if a data mark of a data block in a sector is lost due to a drop-out and the data in the data block contains the same bit pattern as the data mark bit pattern, such a bit pattern in the data would be erroneously detected as a data mark. Also, if a drop-out exists near a data mark in a sector in an adjacent track, the data mark in the adjacent rack would be erroneously detected due to cross-talk. Such erroneous data mark detection will cause the corresponding demodulation circuit to be activated to inadvertently demodulate the data in the sector defected due to drop-out.
It is also understood that a known rewritable optical disk experience an increase in operational error due to the fatigue of its recording layer after a particular number of recording operations. Although a corresponding demodulation circuit identifies a demodulation error such as bias demodulation, it cannot detect any sign of deterioration in the data. Accordingly, erroneous data will be demodulated for reproduction, or the error cannot be corrected when surpassing its limit.